Call function devices that work for all the telephones on the telephone line, such as some speed dialers and call restrictors, are typically connected serially between the user""s telephone and the telephone link to the central telephone office. In the operation of the speed dialer, dual-tone multi-frequency (DTMF) tones generated by the telephone are received by the device. If it is determined that these tones are representative of a speed dialing code, the speed dialer accesses a lookup table using the code as the address. A telephone number stored at the address is then sent over the telephone link, and the telephone is subsequently connected to the link. The keystrokes necessary to dial a full 11-digit telephone number, for example, are cut to the number of keystrokes necessary to type the code, usually three or four keystrokes. Alternatively, if the DTMF tones generated by the telephone are simply a telephone number that a user wishes to call, the speed dialer forwards the tones onto the telephone link and connects the telephone to the link. In this way, these devices add a new capability while still allowing the user to dial normally.
The principle downside of serial-connected speed dialers is the difficulty of installation. Serial connection can be made easily for one telephone, but it presents difficulties for many telephones, since there is often no modular jack available that allows a device to be plugged between the central telephone office and all of the phones on a phone line. This installation often requires an experienced technician, which increases costs.
Parallel-connected call restrictors have been proposed. These devices are usually connected in parallel to telephones across the tip and ring lines of the telephone link to block certain dialing codes. The parallel connection requires no special wiring because the parallel-connected device can plug into a modular jack connected to the phone line and work for all phones on the line. If the desired jack is being used by a telephone, a T-adapter can be used or the device can provide this function. Parallel-connected speed dialers have also been proposed. Such a dialer would be plugged into a modular jack to receive a speed dialing code from other telephones on the link. The parallel-connected dialer then generates a particular phone number that this code indicates.
Known parallel-connected call function devices, such as speed dialers or call restrictors, have suffered from a number of problems. At some point during operation, the device should load the telephone link to ensure that dialing signals generated by one of the telephones are not detected by a central office serving the link. One method for achieving this is to place an a.c. (alternating current) load across the phone link""s two conductors, tip and ring. This a.c. load should severely attenuate the dialing signals on the line. Prior art specifications for the a.c. load, however, are inadequate for many real-world situations. Variability in central office sensitivity, telephone network attenuation, and dialing signal strength means that, in many common situations, the signals generated by the telephone will be detected by the central office.
Moreover, in many cases the call function devices are not transparent to the user. Some familiar aspects of telephone operation change with installation of the device, or the audio amplitude of the link deteriorates. For example, the user may no longer hear a dial tone or the fidelity of the telephone link may be impacted.
In one aspect, the present invention is directed to a parallel-connected device that inhibits the transmission of dialing signals over a telephone link. An a.c. load, preferably a series-connected capacitor and resistor, is provided to inhibit transmission by attenuation of the signals. Specifically, the signals are attenuated by at least 30 dB. This attenuation should ensure that the dialing signals will not initiate action by central offices meeting accepted specifications. In specific embodiments, higher attenuations are preferable. If one considers real-world central offices and worst-case DTMF sensitivities, attenuation of at least 38 dB is preferred.
In other aspects, the design and operation of the invention provide for transparency to the user. For example, in the time between the phone initially going off-hook and the transmission of the first DTMF signal, the user at the telephone is able to hear a normal dial tone with the present invention. This operation is deceptively difficult to achieve since the a.c. load attenuates the dial tone. This fact suggests that the load should be connected across the telephone link in response to the detection of the start of the first dialed digit. On the other hand, an a.c. load that will properly attenuate the dialing signals and that is charged by d.c. current from the central telephone office, only when thus connected, may take too long to charge up. A certain minimum voltage required by the phone""s circuitry for DTMF signal generation may not be available for at least a portion of the time while the capacitor charges so the telephone may not be able to continue to generate the DTMF signal for proper detection. In the present invention, this problem is solved by connecting the a.c. load across the telephone link when a telephone""s off-hook status is first detected. This charges a capacitor, the primary a.c. load. Subsequently the a.c. load is disconnected, allowing the dial tone to reach the user. The a.c. load is then reconnected only when the user begins to dial a telephone number or other code.
Further transparency is provided in certain embodiments by using a single interfacing capacitor to connect both a DTMF detector and a tone generator to the tip line. This reduces the loading on the tip line by the device during periods when users are conversing on the telephone link, thus preserving the audio quality of the telephone link, and also reduces the manufacturing cost of the device.
The invention may operate as a speed dialer generating a telephone number in response to a code entered at the telephone number pad. In other applications, the invention may select a telephone carrier in response to the dialed telephone number and forward the telephone number to the selected telephone carrier. This feature is helpful for intra-LATA toll calls in which long distance carriers often have better rates. Still other applications for the invention include transmitting DTMF signals encoding a credit card number for billing purposes when a predetermined dialing code is entered at the telephone. In another situation, the invention functions as a toll call restrictor by only allowing certain calls, such as local and xe2x80x9c800xe2x80x9d telephone numbers, to be sent to the central office.
In still other embodiments, the invention relies on an equalizing network to provide a dialing signal to the dialing signal detector. Because of the network, these dialing signals have the same input level whether or not the switchable load is connected to or disconnected from the telephone link. The equalizing network voltage divides the signals in response to the state of the load to help ensure that signals of the same magnitude are presented to the detector regardless of the state of the load.
According to another aspect, the invention concerns a system for transmitting information over telephone wiring of a home or office, for example. Such a system has a detector for identifying dialing signals across the tip and ring conductors of the telephone wiring. These dialing signals are typically generated by a conventional telephone also connected to the wiring. A switchable load selectively connects to the tip line to attenuate the dialing signals to prevent action by a central office connected to the telephone wiring, but a controller detects these blocked signals. If they are simply a telephone number, they are forwarded to the central office as described previously. In contrast, when it is determined through the formatting that the dialing signals are intended for the system, they are interpreted as commands.
In specific embodiments, the commands are used to control an appliance, such as a thermostat, lights, etc. In other cases, the controller uses the commands to select an intercom message, or to broadcast a spoken message from the telephone as an intercom message.